1906] SHANTZ—VEGETATION OF THE MESA 183 
The species of the ruderal formation are almost entirely native 
plants which readily invade any area from which the vegetative 
covering has been removed. Of the true ruderals, Salsola Tragus 
pushes its way into the formation proper. Here it occurs as dwarfed 
plants which seldom branch, and which die during the aestival period. 
Leptilon canadense, which occurs only here and there in the ruderal 
formation, also occurs throughout the grass formation. The plants 
are usually reduced to 1o™ in height. 
PLAINS BANK FORMATION. 
Whenever a ditch is made through the grass or other formations, 
or where the grass formation is irrigated, the bank formation comes 
in. It matters not what kind of soil, the presence of an abundance 
of water enables this formation to succeed. © This, however, is not 
true invasion, and comes about only as a result of changed condi- 
tions which make the existence of the grass formation impossible. 
Good examples of the coming in of the bank formation may be found 
in irrigated meadows and in small areas where irrigation ditches or 
reservoirs have leaked. : 
A study of this formation is of the greatest interest, for it is along 
this formation that the eastern species find their way into the region. 
The reason is obvious, for here they find suitable conditions of water 
supply. As a result, it is here that the vegetation is made up of the 
most widely distributed species. 
The mountain species find in this formation the cool temperature 
that enables them to exist away from their natural habitats. These 
species pass down along the brook banks, while the eastern species 
Pass up along these same banks. The result is a varied flora. 
The radical difference in water content between this formation 
and the Bouteloua formation does not permit of direct invasion. 
The following species, however, may occur in either formation: 
Erigeron flagellaris, A gropyron occidentale, Helianthus annuus, HH. 
petiolaris. ; 
The facies of the bank formation varies greatly with the age of 
the formation. In the ultimate stage they are as follows: 
Populus deltoides Marsh., P. angustifolia James, P. acuminata Rydb., 
Salix spp., Prunus melanocarpa (A. Nels.) Rydb., P. americana Marsh., Rosa Sayi 
